Molecular Genetic Study of Stem Cell and Cancer Stem Cell Regulation in Drosophila and Mice Our laboratory is interested in understanding how intercellular signals regulate development in embryo and regeneration in adult. We have been studying the JAK-Stat signaling in Drosophila. From our work and that of many others, it has emerged that the same signaling controls numerous decisions in development and also regulates stem cells in various adult tissues, including testis, kidney, stomach, and intestine. Current work is directed at understanding how adult stem cells are regulated in Drosophila and mouse in vivo genetic systems. Our current projects have evolved from our earlier studies of the JAK-Stat signal transduction pathway in Drosophila. In 1996, I identified mutations in the Drosophila stat gene (working with Dr. Norbert Perrimon in Harvard Medical School) and opened the field to the use of fly genetics to understand the JAK-Stat signaling mechanisms and functions (Hou et al., Cell, 1996). Over the years, my lab in NCI has made several major discoveries in understanding of the JAK-Stat signaling, including the identification of a receptor for the JAK/STAT signal transduction pathway (Chen et al., Genes Dev., 2002) and the discovery that the JAK/STAT pathway and Cyclin D/Cdk4 cooperatively regulate tumor development in fly blood and eye (Chen et al., Dev. Cell, 2003). During a genetic screen for genes that interact with the JAK-Stat signaling in male germline stem cells (GSCs), we discovered that a RapGEF/Rap signaling regulates stem cell anchoring to the niche by regulating E-cadherin-mediated cell adhesion (Wang et al., Dev. Cell, 2006). We also generated mice carrying a conditional knockout of the RapGEF gene and found that a RapGEF/SCL pathway regulates development of haematopoietic stem cells (Satya et al., Blood, 2010). Using a GFP reporter (Stat-GFP) for the JAK-Stat signaling in Drosophila, we found that the signaling is activated in stem cells in several adult tissues and the Stat-GFP reporter can be used as a stem cell marker. Using the stem cell marker we identified adult kidney multipotent renal and nephric stem cells (RNSCs) in the Drosophila Malpighian tubules (MTs) and demonstrated that an autocrine JAK/STAT signal regulates the kidney stem cell self-renewal (Singh et al., Cell Stem Cell, 2007). The adult fly kidney stem cells are relatively quiescent and only divide once in one week. However, they can become very active and even develop stem cell tumors upon activating the JAK-STAT signal transduction pathway or expressing the activated form of the Ras oncogene (Singh et al., Cell Stem Cell, 2007;Zeng et al., JCP, 2010). The function and anatomical location of adult kidney stem cells are evolutionarily conserved from fly to fish and maybe also to mammals (Zeng and Hou, Cell Stem Cell, 2011). We recently also identified gastric stem cells (GaSCs) in the adult Drosophila gastric and stomach Organs by using the Stat-GFP reporter (Singh et al., Cell Cycle, 2011). We further found that JAK-STAT signaling regulates GaSC proliferation, Wingless signaling regulates GaSC self-renewal, and Hedgehog signaling regulates GaSC differentiation. The differentiation pattern and genetic control of the Drosophila GaSCs are remarkably similar to what are observed in the mouse gastric stem cells. In all stem cell systems that we have checked so far, a common theme emerges: the JAK-Stat pathway is a major signaling that regulates stem cell proliferation and works in combination with other signals to control stem cell fates. The other signals can be differ in different stem cell systems. For example, the JAK-Stat signaling collaborates with the Notch signaling in posterior midgut intestine stem cells (ISCs), with the Ras/Raf signaling in RNSCs, and with the Wingless and Hedgehog signaling in GaSCs. The Stat-GFP is a general stem cell marker;escargot (esg) is a stem cell marker in GSCs, ISCs, and RNSCs;wingless (wg) and patched (ptc) are stem cell markers in GaSCs. Using these stem cell markers, we are performing genetic screens and have so far identified a number of novel regulators of GSCs, ISCs, RNSCs, and GaSCs. Molecular and genetic characterization of several novel regulators is ongoing. Reporters of the JAK-Stat signaling and Wg signaling are stem cell markers in several Drosophila tissues. These signalings also regulate stem cells in several mammalian systems. We reasoned that these reporters might be stem cell markers in mice. We are currently developing several reporter mice of the JAK-Stat and Wnt signaling, which will drive GFP and CreER in the target cells. We plan to study adult stem cells (particularly in kidney and stomach) in mice by using the new mice lines. Our investigation and knowledge of stem cells in Drosophila should help analysis of stem cells in mice. In summary, we have already laid the groundwork for both fly and mouse projects, and expect to reap the fruits in the next few years. Using this cross-species approach we expect both to obtain an enhanced understanding of stem-cell regulation and to identify new targets for the treatment of human diseases.